There is a growing interest in organic plastic, ophthalmic lens members formed from polymers having refractive indices of at least 1.56. The lens member may be an integral, monofocal lens that provides single vision correction. Presently, the predominant organic plastic material for such a lens is a thermosetting, polycarbonate resin commercially available from PPG Industries, Pittsburgh, Pa. under the trademark CR-39. This material provides the standard refractive index of about 1.50.
Alternatively, the lens member may be a segment adapted to use in conjunction with a major lens member of lower refractive index to provide a composite multifocal lens. U.S. Pat. No. 4,793,703 (Fretz, Jr.), and pending U.S. patent application Ser. No. 07/325,880, filed Mar. 20, 1989 in the name of E. R. Fretz, Jr. and assigned to the assignee of this application, now U.S. Pat. No. 5,064,712 describe a three-layer, composite lens structure. The basic structure disclosed there is composed of an inorganic glass layer, preferably a photochromic glass, a layer of a rigid, organic plastic and an interlayer of a flexible, organic adhesive.
A characteristic feature of the structure in the patent is the relationship between the coefficient of thermal expansion of the rigid, plastic layer and the glass layer. A feature of the structure in the application is the provision of chemically reactive, organic groups on the surfaces of the layers to enhance bonding of the layers.
Multifocal lenses employ different surface curvatures, and/or materials having different refractive indices, to provide multiple visual correction. Thus, the major lens portion, or element, in a multifocal lens corrects far vision. The segment portion provides near vision correction, that is, closer focusing for reading or other close work. The segment is generally small, and located in the lower half, toward the inside, of a major lens. The most functional and widely accepted shape for the bifocal segment is referred to as a D-segment. This appears in the finished lens as a circle with the top cut off.
Multifocal lenses have long been produced in all-glass structures. Thus, a glass element, having a relatively high refractive index and commonly referred to as a segment or button, is fused in a depression formed in a full-sized lens member, known as a major or crown. The depression is usually termed a countersink. The constituent elements of, and method of producing, an all-glass, multifocal lens structure are described in some detail in U.S. Pat. No. 4,842,632 (Buckley).
All-plastic, multifocal lens structures are also available. U.S. Pat. No. 4,906,422 (Buckley) describes moulds for use in producing such lens structures. The conventional all-plastic, multifocal lens is an integral structure. Higher refraction is achieved by moulding a convex lens shape on the front of the unitary lens. Customarily then, the near vision segment in a prior, all-plastic, multifocal lens protrudes from the front of the lens.
An all-plastic lens has the distinct advantage of light weight. A glass lens is harder, and hence more resistant to scratches and other abrasive damage. Hence, there has long been a desire to effect a marriage of a scratch resistant, glass, front lens with a light weight, organic plastic, rear, or backing, lens. The Fretz, Jr. disclosures are directed to meeting this need, but do not specifically disclose a multifocal structure and production method.